In general, a pressure control valve (which means a relief valve) is used to form a set pressure in a hydraulic circuit. In the case of replacing a working device depending on a work condition, the set pressure of the relief valve is reset depending on the working device.
Referring to FIG. 1, a pressure control valve in the related art includes a sleeve having a high-pressure flow path formed on one side thereof to pass high-pressure hydraulic fluid from a hydraulic pump and a hydraulic tank side flow path formed on a circumference thereof to drain the hydraulic fluid in the high-pressure flow path to the hydraulic tank; a poppet 20 controlling a flow rate of the hydraulic fluid flowing from the hydraulic pump side to the hydraulic tank side as sliding in the sleeve 10; a poppet spring 30 elastically supporting the poppet 20; a pilot poppet 40 draining the high pressure to the hydraulic tank if the high pressure that exceeds a set value is generated in a pressure chamber formed in the sleeve 10; a valve seat 50 installed to face the poppet 20 so as to form the set pressure in the pressure chamber in the sleeve 10; a pilot piston 60 sliding in the poppet 20 by an initial hydraulic pump pressure to be in contact with the pilot poppet 40; a pilot piston spring 70 elastically supporting the pilot piston 60; a pilot poppet spring 80 elastically supporting the pilot poppet 40, a main piston 100 slidably coupled to the inside of the guide 90 to variably adjust the set pressure in the pressure chamber in accordance with pilot signal pressure; a main piston spring 110 elastically installed in the guide 90 to elastically support the main piston 100; a stopper 120 controlling movement of the main piston 100; a control plug 130 controlling the set pressure of the main piston 100; a fastening member 141 fastened to the guide 90 to prevent a pressure change after setting the set pressure of a pressure control valve; and a fastening member 142 fastened to the control plug 130.
If the hydraulic fluid on the pilot line is pressed through a connection port 150, the hydraulic fluid is introduced at the main piston 100 through a line 2 formed on the guide 90. Through this, the main piston 100 moves in the right direction in the drawing to press the main piston spring 110 that fixedly supports the main piston 100. At the same time, the pilot poppet spring 80 moves in a direction where the elastic force of the pilot poppet spring 80 is lowered, and thus the pressure set value that is applied to the hydraulic pump is decreased to change the pressure set condition. At this time, the stick phenomenon occurs. That is, even if a desired pressure set value is input, the actual pressure does not coincide with the set pressure. This is because the main piston 100 and the stopper 120 are in close contact with each other to achieve a sealing effect between close contact surfaces thereof, and the hydraulic fluid in the main piston 100 is unable to be smoothly drained, but has a force to push the main piston 100 in the left direction in the drawing. Due to the increased force in the left direction in the drawing, pilot pressure to move the main piston 100 in the right direction in the drawing is increased to be the major cause of the initial pressure abnormality.
When the main piston 100 moves for the distance “X” in this state, the hydraulic fluid is not smoothly drained through the gap between the main piston 100 and the stopper 120, and a damping phenomenon occurs at a stroke end portion to lower the control pressure.